The broad objectives of this proposal are to discover new cellular and molecular pathways that are required for biliary system formation utilizing zebrafish as a model system. The biliary system is responsible for transportation of bile from the liver into the intestine, and many human liver diseases are attributed to malfunction of this system. Previous clinical reports have indicated a tight correlation between intrahepatic biliary and vascular network malformations in the diseased liver, however molecular and cellular mechanisms underlying the correlation are poorly understood. Therefore, I will apply well-established zebrafish forward genetics to analyze the interaction between intrahepatic biliary and vascular networks as well as biliary system development. When the patterning of the intrahepatic vascular network is disrupted, the patterning of the intrahepatic vascular network is also changed in zebrafish. This situation in the zebrafish liver mimics that in the human diseased liver, suggesting that studies in zebrafish will help to understand the pathology of human biliary disorders. Indeed, the interaction between the intrahepatic biliary and vascular networks appears to be reciprocal. Therefore, I will first characterize how vascular endothelial cells regulate intrahepatic biliary network formation. I will subsequently characterize how the intraheaptic biliary network regulates intrahapeatic vascular network formation. These studies will provide important criteria for a planned forward mutagenesis screen for genes responsible for the interaction and biliary system formation. The transgenic zebrafish lines which express fluorescent protein in the specific organ allowed efficient large-scale forward mutagenesis screens identifying a diverse array of genes responsible for organ development. Therefore I will generate a new fluorescent transgenic line that marks the biliary system in zebrafish embryos. I will subsequently conduct a large scale forward genetic screen to identify many novel genes responsible for biliary system formation. Thus I propose the following specific aims: 1) investigate the role of vascular endotheilal cells in intrahepatic biliary network formation; 2) investigate the role of the intrahepatic biliary network in intrahepatic vascular network formation; 3) prepare and conduct a forward genetic screen to identify genes responsible for biliary system formation as well as the interaction between inthrapetatic biliary and vascular networks. These studies in zebrafish ultimately may serve as a novel gateway to gain new insight into the cellular and molecular mechanisms of human liver diseases.